The prevailing strain measuring device today is the electrical resistance strain gage. It is made of metallic foil in the form of a geometric pattern. When cemented onto a surface undergoing mechanical stress or strain, the gage's resistance change is read by an instrument such as Wheatstone bridge and the value converted back to strain. It is a contact method in that lead wires are needed to monitor the resistance change. The invention relates to two noncontact stain measuring methods using moire fringes. The traditional moire method of strain measurement uses the fringe spacing (or the gradient of fringe position) as a parameter for calculating the strain. This traditional method is described in several references, such as the 3rd edition of the Society of Experimental Mechanics, Manual on Experimental Stress Analysis, March, 1978-Chapter 6-"Moire Methods of Strain Analysis"-by Fu-Pen Chiang, the applicant of the present invention, Moire Analysis of Strain, by A. J. Durelli & V. J. Parks, Prentice Hall, Inc., 1970, and Moire Fringes in Strain Analysis, by P. S. Theocaris, Pergamon Press, Inc., 1969.
Commonly moire fringes are also used for metrological measurements, such as motion and position, taught in U.S. Pat. No. 3,768,911, displacement, taught in U.S. Pat. No. 4,079,252, or three-dimensional configurations of an object, taught in U.S. Pat. Nos. 4,051,483 and 4,525,858. All these methods are conceptually different from the measurement of strain described hereinbelow.